1. Field of the Invention
The present invention relates to a can lid opened by rupturing a score line, and more particularly, to a can lid (i.e., an easy open end) opened by lifting a tab mounted on an opening piece to propagate the rupture along the score line.
The disclosure of Japanese Patent Applications No. 2005-353951 filed on Dec. 7, 2005, and No. 2003-375160 filed on Nov. 5, 2005, including specification, drawings and claims are incorporated herein by reference in its entirety.
2. Discussion of the Related Art
The can lid of this kind is used in a beverage can or a can containing solid materials such as fruits or meat. As a can lid for the former can containing liquid such as a beverage, a partial open type can lid, in which a panel portion of the can lid opens partially to form an opening, is known in the art. Especially, a stay on tab type easy open can lid is used generally in the conventional art. According to the stay on tab easy open can lid, a can is opened without detaching a tab and an opening piece from the can (or a can lid). On the other hand, the latter can containing solid materials have to be opened widely to eject the solid contents easily. For this purpose, a can lid called a full-open type can lid has been developed. A panel portion of this kind of full-open type can lid is opened widely by rupturing a score line formed around the panel portion of the can lid.
An example of the full-open type can lid is disclosed in Japanese Patent Laid-Open No. 2004-161360. In the can lid disclosed by this publication, the score line is formed annularly. Therefore, when a tab mounted on an opening piece is lifted, the opening piece defined by the score line is ruptured along the score line and the opening piece is detached from the can lid eventually. As a result, an opening can be formed widely. On the other hand, if the can is opened at once, or if the opening piece is detached from the panel portion abruptly, the contents may be scattered out of the can due to an opening shock. For this reason, according to the invention taught by Japanese Patent Laid-Open No. 2004-161360, a residual thickness of the score line in a portion opposite to a portion where the tab is mounted is made thicker to gradually increase rupture strength. Therefore, the propagation of the rupture of the score line is halted just before the termination of opening operation.
Meanwhile, Japanese Patent Laid-Open No. 2004-155480 discloses an example of a can lid whose opening dimension is limited to about half area of a panel portion. According to the can lid suggested by this publication, a score line is formed half round of the panel portion, and a fold guide which is not to be ruptured is formed linearly connecting both ends of the score line. According to the can lid of this kind, the opening piece is folded at the fold guide so that an opening dimension can be substantially widen without the opening piece intervening, even if the opening piece opens only within a half area of the panel portion. Moreover, it is possible to close the lid temporarily by returning the opening piece.
The aforementioned full-open type can lid is capable of opening the panel portion widely so that the contents can be ejected comparatively easily even if the solid materials are contained therein. However, if the opening piece of the full-open type easy open can lid is completely detached from the can lid and thrown away, this becomes a problem in view of resource recovery and environmental conservation. In order to cope with this kind of problem, the annular score line is formed as an incomplete circle, specifically, both ends of the score line are separated at a certain distance to be faced to each other. One example is disclosed in Japanese Utility Model Laid-Open No. H6-47128. According to the full-open type can lid of this kind, the opening piece is kept connected with the can lid at a portion between the both ends of the score line, i.e., at an unscored portion. The unscored portions are formed in a portion of an outer circumference of the panel portion, therefore, the panel portion can be opened almost entirely.
As mentioned above, if the opening piece of the full-open type can lid is completely detached from the can lid, the opening piece is thrown away and this causes a problem in view of resource recovery and environmental conservation. However, the opening piece may be detached due to an extension of a rupture to the unscored portion, even if the unscored portion is formed to keep the opening piece connected with the can lid. Specifically, when the tab is lifted, a portion of the score line is ruptured initially, and then, the rupture is propagated as the tab is pulled backward. Since the rupture along the score line is propagated at an accelerating pace, the rupture may extend to the unscored portion. As a result of this, the unscored portion is ruptured so that the can lid is opened wider than expected or the opening piece is detached completely from the can lid.
The above-mentioned can lids disclosed by individual patent documents are capable of widening the opening dimension, however, a sharp edge is created almost all around the opening portion as a result of the rupture of the score line. The sharp edge is also created almost all around the outer circumference of the opening piece. In order to avoid injury by the sharp edge, an overlapping portion is formed all along the score line.
The overlapping portion, which is also called as a safety portion, is formed by folding a metal sheet forming the panel portion at an outer circumference of a portion to be the opening piece, and at an inner circumference of the opening created as a result of a rupture of the score line. In the opening piece side, the metal sheet is folded into multiple layers to form the overlapping portion on an upper face of the opening piece, and in the opening side, the metal sheet is folded into multiple layers to form the overlapping portion on a lower face of the panel portion (i.e., in an inner side of the can container). The score line is formed on an intermediate portion between those overlapping portions of inner and outer circumferential sides.
In any of aforementioned types of the can lids, it is preferable to rupture the score line gradually when open the can lid. For this purpose, it is preferable to open the lid while folding or bending the opening piece. However, section modulus of aforementioned overlapping portion is high so that stiffness thereof is enhanced. This makes the opening piece hard to be folded or bent. As a result of this, the score line is ruptured abruptly and the contents may be scattered out of the can.
Further, in case an inner pressure of the can container rises, the panel portion or the opening piece is inflated into a domed shape, and such deformation of the panel portion or the opening piece may not be restored even if the inner pressure is lowered as a result of an opening operation. This kind of deformation may interrupt the folding or bending of the opening piece, therefore, a bead portion may be formed on the opening piece so as to avoid such deformation. For example, the bead portion is formed by recessing a portion of the opening piece linearly, however, this increases the stiffness of the opening piece itself. The high stiffness of the opening piece and the overlapping portion makes the opening piece hard to be folded or bent when open the can lid. As a result of this, the score line is ruptured abruptly and the contents may be scattered out of the can.